Yeah, it's 100% 了
The font made it kinda hard to tell. I would've expected it to look like the 子 a few lines below it, just without the middle horizontal line, but the tiny font tile makes it look completely different, shifted to the right and shorter on top.
The tone of the first example line you gave made me think military-ish, so they'd probably say 終了 and 了解 and stuff, but I don't understand what they're actually saying, so I couldn't be sure from that alone.
Every additional sample you shared is 了解 了かい or "roger" so I'm confident it's correct now.